Is there a need for a new API for yielding MONITOR/MWAIT to the guest?
Why not just tie this to the guest CPUID.01H:ECX[MWAIT] being set?

On Mon, Mar 12, 2018 at 4:53 AM, Wanpeng Li <kernel...@gmail.com> wrote:
> From: Wanpeng Li <wanpen...@tencent.com>
>
> Allowing a guest to execute MWAIT without interception enables a guest
> to put a (physical) CPU into a power saving state, where it takes
> longer to return from than what may be desired by the host.
>
> Don't give a guest that power over a host by default. (Especially,
> since nothing prevents a guest from using MWAIT even when it is not
> advertised via CPUID.)
>
> Cc: Paolo Bonzini <pbonz...@redhat.com>
> Cc: Radim Krčmář <rkrc...@redhat.com>
> Cc: Jan H. Schönherr <jscho...@amazon.de>
> Signed-off-by: Wanpeng Li <wanpen...@tencent.com>
> ---
>  Documentation/virtual/kvm/api.txt | 23 ++++++++++++++---------
>  arch/x86/include/asm/kvm_host.h   |  2 ++
>  arch/x86/kvm/svm.c                |  2 +-
>  arch/x86/kvm/vmx.c                |  9 +++++----
>  arch/x86/kvm/x86.c                | 24 ++++++++++++++++++++----
>  arch/x86/kvm/x86.h                | 10 +++++-----
>  include/uapi/linux/kvm.h          |  2 +-
>  tools/include/uapi/linux/kvm.h    |  2 +-
>  8 files changed, 49 insertions(+), 25 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt 
> b/Documentation/virtual/kvm/api.txt
> index 98de506..76e5a15 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4358,6 +4358,20 @@ enables QEMU to build error log and branch to guest 
> kernel registered
>  machine check handling routine. Without this capability KVM will
>  branch to guests' 0x200 interrupt vector.
>
> +7.13 KVM_CAP_X86_DISABLE_EXITS
> +
> +Architectures: x86
> +Parameters: args[0] defines which exits are disabled
> +Returns: 0 on success, -EINVAL when args[0] contains invalid exits
> +
> +Valid exits in args[0] are
> +
> +#define KVM_X86_DISABLE_EXITS_MWAIT            (1 << 0)
> +
> +Enabling this capability on a VM provides userspace with a way to no
> +longer intercepts some instructions for improved latency in some
> +workloads.
> +
>  8. Other capabilities.
>  ----------------------
>
> @@ -4470,15 +4484,6 @@ reserved.
>      Both registers and addresses are 64-bits wide.
>      It will be possible to run 64-bit or 32-bit guest code.
>
> -8.8 KVM_CAP_X86_GUEST_MWAIT
> -
> -Architectures: x86
> -
> -This capability indicates that guest using memory monotoring instructions
> -(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit.  As such 
> time
> -spent while virtual CPU is halted in this way will then be accounted for as
> -guest running time on the host (as opposed to e.g. HLT).
> -
>  8.9 KVM_CAP_ARM_USER_IRQ
>
>  Architectures: arm, arm64
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0395c35..e107171 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -811,6 +811,8 @@ struct kvm_arch {
>
>         gpa_t wall_clock;
>
> +       bool mwait_in_guest;
> +
>         bool ept_identity_pagetable_done;
>         gpa_t ept_identity_map_addr;
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index be9c839..321b3fd 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1390,7 +1390,7 @@ static void init_vmcb(struct vcpu_svm *svm)
>         set_intercept(svm, INTERCEPT_XSETBV);
>         set_intercept(svm, INTERCEPT_RSM);
>
> -       if (!kvm_mwait_in_guest()) {
> +       if (!kvm_mwait_in_guest(svm->vcpu.kvm)) {
>                 set_intercept(svm, INTERCEPT_MONITOR);
>                 set_intercept(svm, INTERCEPT_MWAIT);
>         }
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 6cefd7b..2302ae2 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3733,13 +3733,11 @@ static __init int setup_vmcs_config(struct 
> vmcs_config *vmcs_conf)
>               CPU_BASED_UNCOND_IO_EXITING |
>               CPU_BASED_MOV_DR_EXITING |
>               CPU_BASED_USE_TSC_OFFSETING |
> +             CPU_BASED_MWAIT_EXITING |
> +             CPU_BASED_MONITOR_EXITING |
>               CPU_BASED_INVLPG_EXITING |
>               CPU_BASED_RDPMC_EXITING;
>
> -       if (!kvm_mwait_in_guest())
> -               min |= CPU_BASED_MWAIT_EXITING |
> -                       CPU_BASED_MONITOR_EXITING;
> -
>         opt = CPU_BASED_TPR_SHADOW |
>               CPU_BASED_USE_MSR_BITMAPS |
>               CPU_BASED_ACTIVATE_SECONDARY_CONTROLS;
> @@ -5531,6 +5529,9 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
>                 exec_control |= CPU_BASED_CR3_STORE_EXITING |
>                                 CPU_BASED_CR3_LOAD_EXITING  |
>                                 CPU_BASED_INVLPG_EXITING;
> +       if (kvm_mwait_in_guest(vmx->vcpu.kvm))
> +               exec_control &= ~(CPU_BASED_MWAIT_EXITING |
> +                               CPU_BASED_MONITOR_EXITING);
>         return exec_control;
>  }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 36ef3d8..5fae476 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2809,9 +2809,15 @@ static int msr_io(struct kvm_vcpu *vcpu, struct 
> kvm_msrs __user *user_msrs,
>         return r;
>  }
>
> +static inline bool kvm_can_mwait_in_guest(void)
> +{
> +       return boot_cpu_has(X86_FEATURE_MWAIT) &&
> +               !boot_cpu_has_bug(X86_BUG_MONITOR);
> +}
> +
>  int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  {
> -       int r;
> +       int r = 0;
>
>         switch (ext) {
>         case KVM_CAP_IRQCHIP:
> @@ -2867,8 +2873,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long 
> ext)
>         case KVM_CAP_ADJUST_CLOCK:
>                 r = KVM_CLOCK_TSC_STABLE;
>                 break;
> -       case KVM_CAP_X86_GUEST_MWAIT:
> -               r = kvm_mwait_in_guest();
> +       case KVM_CAP_X86_DISABLE_EXITS:
> +               if(kvm_can_mwait_in_guest())
> +                       r |= KVM_X86_DISABLE_EXITS_MWAIT;
>                 break;
>         case KVM_CAP_X86_SMM:
>                 /* SMBASE is usually relocated above 1M on modern chipsets,
> @@ -2909,7 +2916,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long 
> ext)
>                 r = KVM_X2APIC_API_VALID_FLAGS;
>                 break;
>         default:
> -               r = 0;
>                 break;
>         }
>         return r;
> @@ -4214,6 +4220,16 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>
>                 r = 0;
>                 break;
> +       case KVM_CAP_X86_DISABLE_EXITS:
> +               r = -EINVAL;
> +               if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
> +                       break;
> +
> +               if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
> +                       kvm_can_mwait_in_guest())
> +                       kvm->arch.mwait_in_guest = true;
> +               r = 0;
> +               break;
>         default:
>                 r = -EINVAL;
>                 break;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index b91215d..cd1215e 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -2,8 +2,6 @@
>  #ifndef ARCH_X86_KVM_X86_H
>  #define ARCH_X86_KVM_X86_H
>
> -#include <asm/processor.h>
> -#include <asm/mwait.h>
>  #include <linux/kvm_host.h>
>  #include <asm/pvclock.h>
>  #include "kvm_cache_regs.h"
> @@ -264,10 +262,12 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, 
> u64 nsec)
>             __rem;                                              \
>          })
>
> -static inline bool kvm_mwait_in_guest(void)
> +#define KVM_X86_DISABLE_EXITS_MWAIT          (1 << 0)
> +#define KVM_X86_DISABLE_VALID_EXITS          (KVM_X86_DISABLE_EXITS_MWAIT)
> +
> +static inline bool kvm_mwait_in_guest(struct kvm *kvm)
>  {
> -       return boot_cpu_has(X86_FEATURE_MWAIT) &&
> -               !boot_cpu_has_bug(X86_BUG_MONITOR);
> +       return kvm->arch.mwait_in_guest;
>  }
>
>  #endif
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 088c2c9..1065006 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -929,7 +929,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_S390_GS 140
>  #define KVM_CAP_S390_AIS 141
>  #define KVM_CAP_SPAPR_TCE_VFIO 142
> -#define KVM_CAP_X86_GUEST_MWAIT 143
> +#define KVM_CAP_X86_DISABLE_EXITS 143
>  #define KVM_CAP_ARM_USER_IRQ 144
>  #define KVM_CAP_S390_CMMA_MIGRATION 145
>  #define KVM_CAP_PPC_FWNMI 146
> diff --git a/tools/include/uapi/linux/kvm.h b/tools/include/uapi/linux/kvm.h
> index 0fb5ef9..b13c257 100644
> --- a/tools/include/uapi/linux/kvm.h
> +++ b/tools/include/uapi/linux/kvm.h
> @@ -924,7 +924,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_S390_GS 140
>  #define KVM_CAP_S390_AIS 141
>  #define KVM_CAP_SPAPR_TCE_VFIO 142
> -#define KVM_CAP_X86_GUEST_MWAIT 143
> +#define KVM_CAP_X86_DISABLE_EXITS 143
>  #define KVM_CAP_ARM_USER_IRQ 144
>  #define KVM_CAP_S390_CMMA_MIGRATION 145
>  #define KVM_CAP_PPC_FWNMI 146
> --
> 2.7.4
>

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